An example of a portable transceiver of the type discussed herein is a two-way pager that can receive messages from a base station transmitter, and also send messages to a base station receiver. Messages sent by the transceiver are generally of two kinds: scheduled messages and unscheduled messages. An example of a scheduled message is an acknowledgment signal that is automatically transmitted by the transceiver to signify that it has received a message sent by a base station transmitter. This form of scheduled message is transmitted on a channel selected by the base station and is sent at a time dictated by the signaling protocol used by the messaging system.
An unscheduled transmission is a message that originates with the transceiver and is sent to the base station during a time interval that is set aside for such messages. In a typical messaging system, the transceiver can select from one of several channels to use for sending an unscheduled transmission. Each such channel has its own frequency, and its data rate may be different from the data rate of the other available channels. One of these channels is selected by the transceiver in a somewhat random fashion according to a program built into the transceiver. The selected channel and its associated data rate will be used to send the unscheduled message, regardless of the condition of the transceiver or the suitability of the selected channel. For example, a selected channel may have a relatively poor signal-to-noise ratio. Consequently, a message sent on that channel by the transceiver may not be received properly. When the transceiver fails to receive an acknowledgment that its message was received, it may then repeat its efforts to send the message on the same channel.
As a result of this and other problems discussed later, messages sent by the transceiver may be severely delayed, or possibly never received. This result is obviously unacceptable to the user of the transceiver and to the operator of the messaging system.